This schedules the job to run if checked, or prevents it from being run if unchecked

SMS Before

This determines how long prior to an appointment that the reminder will be sent. It will never be sent before this, but may be sent after, if the Reminder Sender did not run when the reminder first became due.

No SMS Before

This prevents the Reminder Sender sending reminders for appointments that are in the near future, i.e. appointments which start less than the specified interval ahead in time. It must be less than the SMS Before interval.
For both the above, an interval of N days means Nx24 hours and similarly a week means 7x24 hours. Thus if the job runs at 13:00, an interval of 1 day covers the period up to 13:00 the next day, not 23:59 the next day.

No Reminder

This interval is checked when the appointment is created. If the appointment is less than this interval in the future, then the Send Reminder flag will not be set for the appointment.